How do I highlight unwelded UV vertex?
Found a way to do what you wanted, but it takes some steps and a addon, TexTools for Blender:

https://blenderartists.org/t/textools-for-blender/700811

http://renderhjs.net/textools/blender/

Place the Addon into: C:\Users\YOURUSERNAME\AppData\Roaming\Blender Foundation\Blender\2.79\scripts\addons, so it can appear in your addon list in blender.

Here's the steps if you wanted to do this way, we're essentially just creating a mesh from the uv map, extruding it, and applying a subdivision surface modifier, where the mesh get's crumppled it's a breaking point in the UV.

So, step 1, gather your mesh and uv map together.



Step 2, using the addon, in the uv editor apply the function, Create UV Mesh.



Step 3, show it in solid form, by going in the object properties and setting the display mode.



Step 4, select the new mesh and extrude it.



Step 5, apply the subdivision modifier to see where the mesh breaks, that's a UV point break.


Step 6, using the information give on step 5, check the uv map where the mesh breaked on the subdivision modifier. There are your UV points break highlighted.


A ugly 6 steps solution, but it works.

The add-on is highly recommended if you like more tools for UVing, just a tip,

Cheers,